Owing to substantial growth of usage in fossil energy while which energy is drying up gradually, to pernicious substances harmful to human bodies produced by extensive application of fossil energy, such as SO2, CO, NOx, and to global climate changes caused by the greenhouse effect due to considerable quantity of exhausted CO2, the world is devoted to the development of new energy technologies. In particular, hydrogen energy is planned to be one of the major energies in the future by the International Energy Agency (IEA), because the byproduct thereof is water only, without CO2, which completely prevents pollution and the greenhouse effect. However, in practical applications, due to the light molecular weight of hydrogen, the storage volume will be immensely huge. Though super-high pressure can be adopted for storage, safety will be another issue.
The problems of storage density and safety of hydrogen are not solved until 1980 when the hydrogen storage alloys that can stores hydrogen in solid state is introduced. Nevertheless, the hydrogen storage density of current commercial hydrogen storage alloys, including transition-metal-based hydrogen storage alloys AB2 or rare-earth-metal-based hydrogen storage alloys AB5, is still too low, less than 2.0% in weight. Thereby, the research and development of high-capacity hydrogen storage alloys is the current international trend. Particularly, magnesium-based hydrogen storage alloys are regarded as potential hydrogen storage alloys due to their low costs in raw materials. However, because pure magnesium is very active, the surface thereof tends to form an oxidation layer that can block absorption of hydrogen molecules, and hence affect diffusion rate of hydrogen atoms on the surface of alloys. As a result, pure magnesium is difficult to be activated and has bad hydrogen absorption-desorption dynamics. In addition, the temperatures of hydrogen absorption and desorption are too high. Accordingly, it cannot be developed to be a practical hydrogen storage alloy.
Regarding to the issue of bad hydrogen absorption-desorption dynamics of pure magnesium, by many researches, it is discovered that by adding nickel with catalyzing effect, the reaction rate of hydrogen absorption-desorption in the hydrogen storage alloy Mg—Ni can be improved, and the initial activation properties is catalyzed as well. In the Mg—Ni-based hydrogen storage alloys, Mg2Ni in the γ-phase has the fastest activation reaction rate and the best hydrogen absorption-desorption property.
Because the melting points of magnesium (649° C.) and nickel (1455° C.) differ greatly, melting tends to be ununiform, which would result in ununiformity in composition of the hydrogen storage alloy. In addition, the vapor pressure of magnesium is high, thereby magnesium is easy to vaporize during melting, which causes severe deviation in initial composition, and excess eutectic structure and formation of the β-phase MgNi2, which is incapable of absorbing hydrogen. In order to solve the problem the severe deviation in composition during melting as described above, next-generation vacuum induction furnaces are introduced. However, although the vacuum induction furnaces are equipped with in-situ inspection, for the hydrogen storage alloy Mg—Ni, owing to its natural characteristic in the phase diagram, the melt liquid of Mg—Ni still cannot give 100%-pure γ-phase Mg2Ni after solidification, even the composition of magnesium and nickel are controlled to be accurately 2:1 via the most precise in-situ inspection function. This is because according to the binary equilibrium phase diagram of magnesium and nickel, in such a composition, far above the melting point 761° C. of the γ-phase Mg2Ni, the β-phase MgNi2, which has a meting point of 1147° C. and is incapable of absorbing hydrogen, has solidified and precipitated first. Besides, because the composition of the β-phase MgNi2 has much more nickel than the γ-phase Mg2Ni, the residual Mg—Ni melt liquid yet solidified deviates from the original composition of the γ-phase Mg2Ni with a magnesium-to-nickel atomic ratio of 2:1, and becomes a magnesium-rich state. The Mg—Ni melt liquid in the magnesium-rich state, according to the binary equilibrium phase diagram of magnesium and nickel, not only will form the γ-phase Mg2Ni if the temperature is lower than 761° C. in the present composition, but also will give an eutectic structure including the pure-magnesium phase at the eutectic temperature of 507° C. That is to say, even the macroscopic composition complies with the proportion of the γ phase, the microscopic structure thereof includes the β-phase MgNi2 and the solid solution phase of pure-magnesium in the γ-phase Mg2Ni. Thereby, the smelt method according to the prior art cannot be used for preparing stoichiometric Mg2Ni compound with fast activation reaction rate and with excellent hydrogen absorption and desorption properties.
Accordingly, the authors of the present invention make advantage of the segregation principle in physical metallurgy, in a broad range of composition and in low temperatures (far lower than the melting point of pure nickel), and propose a simple apparatus for continuously manufacturing stoichiometric Mg2Ni compound.